1. Field of the Invention
This invention relates generally to offshore floating vessels and platform structures used in the exploration and production of oil and gas products, and more particularly to an offshore floating platform structure having vertical columns connected at a lower end by lateral trusses, a telescoping keel tank supported beneath the columns, and a rectangular ring-like deck mount structure or a three-sided deck mount structure at the top of the columns open on one side to allow on-site float-over deck installation.
2. Brief Description of the Prior Art
In the following discussion the term “truss”, as used herein, refers to a welded or bolted cross braced open frame structure formed of slender tubular members. A truss bridges between vertical column structures to stabilize a semi-submersible vessel at the water surface when floating with respect to wind, wave, current and other horizontal loads. As used herein, the term “moment connection” means a connection designed to transfer moment as well as axial and shear forces between connecting members. The term “pin connection” means a connection designed to transfer axial and shear forces between connection members, but not moments.
Floating vessels and semi-submersible floating vessels, such as floating production platforms, storage and offloading vessels, tension leg platforms (TLPs) and SPAR structures, are commonly used for oil well drilling, oil production and living and working quarters. It is desirable to design floating structures with minimum heave (vertical) oscillations to waves in the ocean environment.
Conventional column-stabilized semi-submersible vessel or platforms typically comprise three or more large diameter vertical columns that are spatially separated and connected at their bottom ends by large horizontal pontoons. The columns and pontoons of modern semi-submersibles are usually constructed of shells formed of thin metal plates backed with welded stiffeners, frames, stringers, bulkheads and stiffeners and may have compartments or voids for ballast and storage. The deck structure is above the water with a sufficient distance between the still water level to the bottom of the deck to allow waves to pass across the columns without impacting on the bottom of the deck. The center of gravity of the entire semi-submersible vessel is generally high due to large deck loads above the water. The columns and the pontoons provide stability and the necessary upward buoyancy required to support the structure, the downward payload of the deck and equipment, and live loads. The center of gravity (CG) of a conventional semi-submersible vessel is usually maintained above the center of buoyancy (CB), unlike a SPAR structure. The center of gravity (CG) positioning controls the roll and pitch period of the vessel and also the vessel stability. Due to the shallow draft of the columns, bringing the center of gravity (CG) below the center of buoyancy (CB) demands a large amount of ballast compensation in the pontoons, in addition to the buoyancy required to support the deck. Thus a conventional semi-submersible requires large water displacement to support the deck payload. Other problems with conventional semi-submersible platforms is that they are not well suited for dry tree support because their heave oscillation varies from a small magnitude in calm sea or small wave conditions to large in stormy rough sea or high wave conditions, the added mass and ballast mass of the pontoon is too large to effectively shift the natural period away from the calm wave period, and damping is very poor and predominantly radial in nature. Thus, the conventional semi submersible platforms may be acceptable for dry-tree support in low and moderate sea states but not in extreme sea states.
A conventional semi-submersible structure is structurally stable in severe wave environment due to the fact that the conventional boxed shell pontoons, either all around or on two sides, provide the vessel with a strong “moment connection” at the bottom of the columns at their bottom ends. The deck structure is typically simply supported or placed at the top of the columns and is a hinge or “pin connection” at the top of the columns and has no capacity to transfer moment to the columns through the connection. In the conventional semi-submersible, the pontoon predominantly provides the required buoyancy and the columns are separated and sized for column stabilized requirements. The pontoon mass is also large to accommodate a large volume of ballast water required to lower the center of gravity (CG) sufficiently to provide adequate stability in extreme sea conditions.
The wave forces are large on the pontoon because the large volume and mass is located at a shallow draft. Thus, the moment connection at the bottom between the columns and the pontoon are subjected to these wave forces and the connection is also subjected to severe storm loadings and fatigue loadings.
My previous U.S. Pat. No. 6,671,124, which is hereby incorporated herein by reference, discloses column-stabilized floating structures having a plurality of vertical buoyant caissons bridged together in distantly spaced relation by a plurality of open frame horizontal truss pontoon members and vertical truss columns at a lower end. A work deck is secured to the top ends of the vertical caissons. The buoyancy of the caissons is selectively adjusted by means of ballast control. Water is selectively pumped into or out of keel tanks at the bottom of the truss structure such that the water mass and weight is adjustably tuned to raise or lower the center of gravity of the entire mass of the floating structure relative to its center of buoyancy.
My previous U.S. Pat. No. 6,899,492, which is hereby incorporated herein by reference, discloses jacket frame floating structures comprising one or more elongate vertical support columns formed of an open cross-braced jacket formwork of tubular members interconnected together and at least one cylindrical buoyancy capsule disposed in the open framework near an upper end and at least one cylindrical second buoyancy capsule near a lower end in vertically spaced relation. The buoyancy capsule(s) may be a single, or a plurality of upper and lower capsules bundled in circumferentially spaced relation with a central opening therethrough. Alternatively, a keel tank may replace the lower capsule. The buoyancy of the upper buoyancy capsule(s) is adjustably tuned to provide a buoyant force and a sufficient water plane area and moment of inertia required for stability of the floating structure, and the water mass and weight of the lower buoyancy capsule(s) or keel tank(s) is adjustably tuned to raise or lower the center of gravity of the entire mass of the floating structure with respect to its center of buoyancy.
My previous U.S. Pat. No. 6,942,427, which is hereby incorporated herein by reference, discloses floating offshore fluid storage caisson platforms having a large diameter vertically oriented buoyant column or caisson, or multiple caissons, defining a storage chamber, and a telescopic keel tank disposed at the bottom end thereof, and may have deck on top of the caisson(s). The structure can be transported horizontally either dry on a transporting vessel or towed with its keel tank in a fully retracted position. At the field of operation, the structure initially floats horizontally. The keel tank is extended and then slowly flooded to move the center of gravity of the structure toward the keel tank and with the heavier tank, the structure tilts upright to assume an operating vertical position with the telescopic keel tank extended downward with respect to the caisson, and thereafter as the storage chamber is filled with fluid, the relative position of the keel tank is adjustably tuned to raise or lower the center of gravity of the entire mass of the structure with respect to its center of buoyancy and maintain the center of gravity of the structure below its center of buoyancy and stabilize the structure vertically at a desired draft.